This invention relates to steam turbines and, more particularly, to a method of assembling side entry control stage blades to a rotor of a steam turbine.
Control stage blades operating in a partial-arc admission steam turbine receive a shock load at the entrance and exit of each admission zone. A single blade is not sufficiently strong to withstand such repetitive shock load forces. Therefore, control stage turbine blades are generally joined into predetermined groups so that as each blade successively receives a shock load, the load is distributed over the entire group of blades. Because control stage blades are subjected to such shock loading forces and are joined in groups, the assembly or installation of such blades into a turbine requires different procedures than for other blades.
It has become common practice to join individual control stage blades into groups of blades by attachment of the blades to a common platform and/or to a common shroud portion. Such multiple blade units have higher rigidity and lower vibration susceptibility than single blades. In one form, a blade group may be constructed by attaching radially outer ends of several blades to a shroud after the blade roots are inserted into their respective rotor grooves. In another form, blade groups may be constructed as integral units having a common shroud and a common platform. Such a blade group is illustrated in U.S. Pat. No. 4,130,379 to Partington and assigned to the assignee of the present invention. These prior art assemblies, however, do not provide for the assembly of individual blades with tight contact between the blades within a group while providing thermal expansion clearances between groups of control stage blades.
For ease of assembly, the individual blades are presently assembled with clearances between the covers and the platforms. The shroud is then riveted to the top of the covers to connect the blades together. A disadvantage to this assembly is that slight movements between the cover and the shroud due to partial admission shock loads prevent even distribution of shock load forces to blades in a group. Uneven distribution of shock load forces may cause excessive vibratory root bending stresses and root cracking. Clearances between covers and platforms also result in lower efficiency due to leakage.